Scissors jacks for raising automotive vehicles or other loads away from the ground are known. Such scissors jacks carry extremely heavy and bulky loads compared to their relatively small size. An example of such a prior art scissors jack is shown in FIG. 1 at 10.
Typically the scissors jacks have two upper channels 11 and 12 joined together by pivot pins 8 and 9 to a U-shaped load bearing member 13. At ends of the U-shaped channels 11 and 12 meshing gear teeth 19 and 20 or 19a and 20a are provided. Lower channels 14 and 15 respectively connect to the upper channels 11 and 12 by pivot pins 7 and 8. The lower channels 14 and 15 also have gear teeth 17 and 18 which intermesh. The lower channels 14 and 15 are connected to an approximately U-shaped base 16 by respective pivot pins 26 and 27 passing through side walls 33 and 34 of the base 16. A threaded screw 22 having one end passing through a central portion of a pivot pin 8 has at that end a crank arm 5 and at the opposite end a threaded engagement in threaded opening 21a of a nut 21b in extended member 21 connected at pivot pin 7. The scissors jack is thus raised and lowered by cranking the crank arm 5.
FIG. 2 shows a top view of the base 16 and the details of engagement of the gear teeth 17 and 18 at one side of the lower channels 14 and 15 and similar gear teeth 23 and 24 engaging at the other side of the lower channels 14 and 15.
When large bulky loads are placed on the load bearing member 13, frequently a twisting action can occur due to unbalance of the load. This twisting action can cause the teeth such as 18 or 24 to disengage from the respective teeth 17 or 23 as shown by the dashed lines at 18', 24' and 17', 23'. This can cause failure of the jack, an obviously dangerous situation where large loads are involved, such as for an automotive vehicle. Such failure might occur while a user of the jack is changing a tire on the automotive vehicle, for example. Also, failure might occur while the user is raising the jack.
In one attempt at a prior art solution to the above described problem, a stamped steel tubular spacer or a piece of steel tubing was placed around the pivot pins 26 and 27 as shown in dashed lines at 3 and 4 in FIG. 2, in an effort to prevent the gear teeth from deforming in an inward direction during extreme loading. However, such an approach adds significant cost and a very significant constructional complexity to the jack assembly.
In another prior art approach to solve the problem, it has been known to form extensions from one sidewall of the lower channels to act as a spacer as shown at 1 and 2 in FIG. 2, also in dashed lines. However, there is the disadvantage with such an approach that it does not securely prevent deformation of the teeth and also most importantly requires a complicated manufacturing method to form such extensions on the one sidewall of the lower channels.